a) Field of the Invention
The present invention pertains to a manufacturing method for a field emission element, and more particularly to a manufacturing method for a field emission element having a sharp tip using a mold concavity having a sharp cusp.
b) Description of the Related Art
A field emission element is an element (emitter) that emits electrons from a cathode tip having a sharp point by means of the action of an electric field. The sharper the convex point of the cathode tip, the stronger is the electric field concentration produced, and the more possible becomes efficient electron emission. It is also desirable to provide a control electrode that is centered on the cathode tip in order to generate an efficient electric field about the cathode tip.
U.S. Pat. No. 5,334,908 discloses a method for forming a mold concavity having a sharp concave end (cusp) above the concavity by means of forming on the surface of the substrate a concavity having perpendicular side walls, and depositing on the substrate surface a sacrificial film. If the sacrificial film is removed after the cathode layer is deposited above the mold concavity, a sharp convex cathode tip is exposed.
If an electrically conducting layer is formed on the substrate surface and a concavity is then formed which penetrates through the said conducting layer, then it is possible to form a control electrode with the conducting layer that has an opening in it, simultaneously with fabricating the cathode tip. The control electrode need not be limited to one electrode but may be made in a plurality of electrode layers.
The present applicant and his colleagues have proposed various methods for forming mold concavities having sharp cusps. For example, when a sacrificial film is deposited in a concavity having a rectangular cross-section, a sharp cusp is formed when the sacrificial film deposited on the two side walls meets in the middle. However, it is not always easy to control the film thickness or, for that reason, the height of the cusp. Control becomes easy by combining the deposition with a chemical reaction. More specifically, the growth of the film being deposited on the two side walls is stopped before it meets in the middle. After that, the surface of the deposited film is subjected to a chemical reaction and its volume thereby caused to expand so that the reacting film surfaces meet in the middle. By imparting a taper to the side walls on the upper side of the concavity, or causing the cross-sectional shape of the cathode tip to converge in two steps, the precision of fabricating the shape of the tips can be raised.
The formation of the cathode film to the top of the mold concavity was being done by sputtering, etc. When the concave tip of the mold concavity becomes sharp, it becomes very difficult to fill cathode emitter material all the way to the bottom of the sharp form. Voids readily develop when one tries to fill emitter material all the way to the bottom of a sharp form. Even if such filling is possible, the process margin becomes very narrow. As a result, the yield of the field emission element manufacturing process becomes low.
Thus, even if a mold concavity having a sharp cusp can be formed, if emitter material cannot be sufficiently filled all the way to the bottom of the cusp of the said mold concavity, a field emission element exhibiting the desired high performance cannot be realized.